In this tutorial you will learn how to assemble a robot hand. This hand has one degree of freedom per finger, so it is easy to slave them to the same actuator to have a simple grasping hand, possibly with a separate actuator for the thumb.
It’s also very useful to use one 6*10*2.5 bearing to put inside the finger joint during assembly.
Fishing line – use for lacing the fingers and form the joints. You will need a lenght of 40cm for finger top joints, and 50cm for the middle joints.
Elastic – for making spring load mechanisms, for the finger return.
Superglue – a slow setting liquid for gluing the elastics.
Cable – use nylon cables to pull the fingers.
Teflon tube – This can be used under the finger sections to help the action of the cables pulling the fingers.
Tools:
You will need those tools to assemble the hand.
Pliers – to press the bearings in the finger joints. You might find it easier to finish with an adjustable plier, as it has two parallel flats.
Small manual hand drill or power drill – to make clearance in the holes for the fishing lines in case the 3D printing wasn’t good enough. You can also use a power drill at low speed.
You will need two drill sizes : 1mm and 1.5mm.
Lighter – for burning the ends of the fishing lines.
Scissors – for cutting fishing lines and cables.
Marker – for marking a dot on the cables, where they will be cut.
First, insert the flat bearing in a finger joint, this allows to have something to press against :
Take the pliers and press in 2 bearings on each side of the joint :
Repeat this for the 3 joints and 5 fingers to obtain a nice articulated hand :
The finger consists of four sections from wrist to tip – first there is the palm bone, then the first bone, then the second bone and then the fingertip bone. The ligaments should connect the top of the tip to the underside of the first, the underside of the tip to the top of the first, the top of the second to the underside of the palm and the underside of the second to the top of the palm.
These two pairs of crossing ligaments constrain the finger to bend evenly which looks more natural and improves grasping of smooth objects.
Make sure each hole has clearance for the fishing lines :
Prepare the fishing lines, 2*50cm for each finger :
Start the lacing on the top of one finger, burn one end to make a small ball, and pull all the way through. The ball should be big enough to stop the cable going in the hole :
Continue the lacing by following the path :
At the end you should come back to the top of the finger tip where you started. At this point mark the cable where you want to cut it, remove the top loop from the recess to give a bit of lack. Bend it backward, force it a bit and burn the end to make a locking ball, and put back the loop in the recess.
Repeat the same for the other part of the finger, you will see the same pattern :
The finger can easily be extended with a single elastic tendon attached to the back of the second bone section.
If need drill some holes to glue elastics :
Cut and attach the elastics to make finger return :
Here are a few other pictures that might help for more details :
After you finish to put the bearings, ligaments and passive tendons, the last step is to add some cable to pull the fingers.
To help the tendons run smoothly it is best to add PTFE eyelets on the inside of each finger. Without these the fingers will move more jerkily and may have difficulty opening.
You can use 4 times 8mm teflon tubes for the bottom of the fingers (except the thumb).
By attaching a tendon to the underside of each finger tip and an eyelet to the other finger sections the finger can be moved by pulling the tendon by hand or an electric motor with a spool.
If you order it on Shapeways, it comes already pre-assembled. The hand has a lot of details which has a specific function.
